Most of furfural production processes utilize direct method, wherein the catalyst is sulfuric acid, and the majority of the raw materials use corn-cob, some of them use other agriculture residues such as corn stalk, cotton stalk, cotton shell, bagasse and etc. Usually, the raw material is crushed down by a crusher(s) and then added dilute sulfuric acid and stirred by an acid mixer(s) until uniformly mixed. The mixture is transported into a hydrolysis reactor. In the reactor, steam is transported into from the bottom and the hydrolyzed furfural is carried out together with the rising steam to form hydrolyzed furfural vapor. The hydrolyzed furfural vapor is transformed into raw furfural liquor after cooling, which is transported into the distillation column to be distilled. Azeotrope of furfural and water is distilled out from the top of the column. After phase separating, the azeotrope is separated into furfural phase which is named as raw furfural, and water phase which flows back into the column. The wastewater discharged from the bottom of the column which includes water, acetic acid and high boiling residues formed during the hydrolysis process, is the furfural wastewater, or called as column bottom furfural wastewater. The raw furfural is then transported into a refining system after further neutralized to be refined into end product furfural. Wastewater including water and a small quantity of furfural is generated during the refining process.
The main components of the column bottom furfural wastewater which is generated during the furfural production process are acetic acid, organic high boiling residues, a small quantity of furfural, and hydrophobic oil-fat substances. Its PH value ranges from 2 to 3 with COD (Chemical Oxygen Demand) as high as from 16,000 to 20,000 generally. The main components of the wastewater generated during the refining process are water and a small quantity of furfural, wherein the COD is usually above 20,000. Nowadays, the disposal of furfural wastewater has been restricting the development of furfural production because of its high COD and the large quantities of wastewater. Small scale furfural manufactures are not available to establish wastewater disposal facilities. They will not be able to dispose the wastewater completely because of the high cost of wastewater disposal even if they invest and construct the wastewater disposal facilities. While large scale furfural manufactures have to increase the cost of furfural production also because of the relatively high wastewater disposal cost. So many developed countries have turned to simply buy the end product furfural from developing countries instead of manufacturing in their own countries as the wastewater is hard to be disposed. The biological contact oxidation method is mostly utilized to dispose the furfural wastewater in China, wherein the wastewater is neutralized, flocculated and sedimentated with lime milk firstly, and then disposed by anaerobic-aerobic method. There are some knotty problems with this method as follows:
1. The calcium salts dissolved in the water after neutralized by lime milk scale on the surfaces of elastic fillers in the anaerobic pool as temperature promoting so as to reduce the adhesion quantities of anaerobic sludges on the surfaces of the elastic fillers, which gradually deteriorates the effect of the anaerobic disposal. If the sodium carbonate or the sodium hydroxide is used for neutralization instead, the cost of disposal will increase sharply.
2. Unlike the wastewater in liquor industry containing large quantities of nutrients such as carbohydrates, the wastewater herein just has few nutrients for anaerobic bacteria together with that the sludge gradually reduces with the wastewater draining during the wastewater disposal process, so, the sludge should be supplied into the wastewater during the process according to this method.
Since it is under certain temperatures that the anaerobic bacteria has better abilities to dispose organics, this method is not suitable in northeast China where the furfural raw materials are abundant but the climate is very cold. This is one of the reasons why the furfural industry is stagnant in northeast China.
This method requires large investment, large area occupation, high running expense, and usually needs 2-3 steps of anaerobic disposal to completely reach the discharge standard.
Additionally, patent application No. 200410011121.2 disclosed a method that after evaporated with furfural vapor, concentrated and then added with dilute acid, the wastewater was utilized to be mixed with acid. But this method has problems shown below:
There are a large quantity of acetic acid and a small quantity of furfural in the wastewater which form secondary pollution during the wastewater evaporation process, since the acetic acid and furfural thereof can not be recycled.
In summary, there exists serious pollution in traditional technologies for discharging of furfural wastewater. It will cause extreme damages to the environment if the problem is not solved.